[Rails] confusion on respond_to do |format| execution sequence

As a new comer for ruby, I feel confused for this snippet:

respond_to do |format|
format.html # index.html.erb
format.xml { render :xml => @posts }
end

I understand that if format is .html, it will give html response and if
format is .xml, it will give out xml response.

So this block "do |format| ... end" is case switch like in C++. I have
difficulty to understand though in ruby, if the user passing .html, as a
sequential execution (notice in ruby this is not a case switch
statement), what prevent ruby from executing "format.xml { render :xml
=> @posts }" line?

To my feeling (though I know I am wrong for sure), these two lines will
always be executed no matter what kind of format is passed in:

format.html # index.html.erb
format.xml { render :xml => @posts }

Please help.

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